How to compare value in ArrayList?

HelloWorld · #1 (**permalink**) 07-16-2007, 02:45 PM

Anybody knows..?
I'm stuck on getting the element of a certain index in ArrayList and compare them with another element in other index..

I tried the following method in ArrayList, but have no clue how to compare them since it's an Object. I was researching on it and it seems that we need to use compareTo() method, but I personally never use it XD

This is what I'm planning to do:
[code]
ArrayList al = new ArrayList();
if (al.get(index_1) < al.get(index_2) {
......
}
[code]

Lee · #2 (**permalink**) 07-16-2007, 02:53 PM

As you know i am not a java guru but i know how much of a pain this is, i have looked up a little and from what i have seen you need something like:

Code:

//Setup booleans
    boolean[] bArr1 = null;
    boolean[] bArr2 = null;
    boolean b = Arrays.equals(bArr1, bArr2);

//To compare, your boolean b is = to the 2 arrays...
    b = Arrays.equals(array with value 1, array  with value 2);

If they match then the result of b will be true, otherwise it will be false, you can then you b to decide weather to do one thing or the other.

Hope that has helped, i have not tested but i am sure you can

rpgfan3233 · #3 (**permalink**) 07-16-2007, 04:02 PM

I haven't used ArrayList at all, but I have used the compareTo() method quite a lot. compareTo() returns one of three possible values:

-1: value1 < value2
0: value1 = value2
1: value1 > value2

However, you shouldn't need it unless you have a specific class that requires compareTo(), such as String. With char, int, etc., you only need the normal logical comparison operators. Also, after doing my own bit of research, ArrayList should actually be used like this:

Code:

ArrayList<Integer> al = new ArrayList<Integer>();
al.add(4);
al.add(9);
/* Remember that each element is simply a java.lang.Integer, the class
     encapsulating the primitive data type 'int'. */
if (al.get(0) < al.get(1)) {
    System.out.println("Minimum value: " + al.get(0));
    System.out.println("Maximum value: " + al.get(1));
}
else { //al.get(0) >= al.get(1)
    System.out.println("Minimum value: " + al.get(1));
    System.out.println("Maximum value: " + al.get(0));
}

No compareTo is necessary. However, if you really want to use it, it would be like the following:

Code:

if (al.get(0).compareTo(al.get(1)) == -1) { //if (al.get(0) < al.get(1))
    System.out.println("Minimum value: " + al.get(0));
    System.out.println("Maximum value: " + al.get(1));
}
else { //al.get(0) >= al.get(1), or al.get(0).compareTo(al.get(1)) > -1
    System.out.println("Minimum value: " + al.get(1));
    System.out.println("Maximum value: " + al.get(0));
}

I hope this helps!

rpgfan3233

HelloWorld · #4 (**permalink**) 07-16-2007, 06:20 PM

Quote:

Originally Posted by Lee

As you know i am not a java guru but i know how much of a pain this is, i have looked up a little and from what i have seen you need something like:

Code:

//Setup booleans
    boolean[] bArr1 = null;
    boolean[] bArr2 = null;
    boolean b = Arrays.equals(bArr1, bArr2);
 
//To compare, your boolean b is = to the 2 arrays...
    b = Arrays.equals(array with value 1, array  with value 2);

If they match then the result of b will be true, otherwise it will be false, you can then you b to decide weather to do one thing or the other.

Hope that has helped, i have not tested but i am sure you can

Haha, I knew this one already in particular, but in this case, I would compare whether it's equal or not by using .equals on my object. I gave an example that I want to compare whether it's less than some other object that I got from the ArrayList

HelloWorld · #5 (**permalink**) 07-16-2007, 06:24 PM

Code:

ArrayList<Integer> al = new ArrayList<Integer>();
al.add(4);
al.add(9);
/* Remember that each element is simply a java.lang.Integer, the class
     encapsulating the primitive data type 'int'. */
if (al.get(0) < al.get(1)) {
    System.out.println("Minimum value: " + al.get(0));
    System.out.println("Maximum value: " + al.get(1));
}
else { //al.get(0) >= al.get(1)
    System.out.println("Minimum value: " + al.get(1));
    System.out.println("Maximum value: " + al.get(0));
}

For some reason this method doesn't work on my code, but that's probably I should try it out by using ArrayList<Integer> al = new ArrayList(); because currently, I'm only using ArrayList al = new ArrayList();

I've seen that method quite often too, but I'm not sure how does it work if I want to create a method that will take ArrayList<Integer> I tried to do this, but it gave me an error:

Code:

public void meth(ArrayList<Integer> al) {
        .....
}

But I think I need to double check again... thanx though rpgfan3233, helps a lot

Thanx lee for the fast response

rpgfan3233 · #6 (**permalink**) 07-16-2007, 07:05 PM

I'm guessing you are calling meth() from a static method such as main(). If you are calling the meth() method from the main() method, for example, you would need to add the static keyword:

Code:

public static void meth(ArrayList<Integer> al) {
    .....
}

Otherwise, you would get an error like this:
ArrayListTest.java:24: non-static method meth(java.util.ArrayList<java.lang.Inte
ger>) cannot be referenced from a static context
meth(al);
^
1 error
I had called meth(al) from main(). Once I added the static keyword, it was fine.